The present invention relates to a process and to a device for heating particle-filled substrates by simultaneous exposure to microwaves and a direct-current magnetic field. This process may be used for heating such substrates for general purposes. It is particularly suitable for substrates comprising a set or unset adhesive or a hot melt adhesive. Depending upon the embodiment, the process may be used for adhesively bonding the surfaces of articles, wherein an unset adhesive or a hot melt adhesive gives rise to an adhesive bond of the surfaces of the articles as a result of the heating. In another embodiment, the process may be used to detach adhesive bonds, wherein in the case of suitable adhesives, such as for example hot melt adhesives, this may proceed reversibly. On exposure to microwave radiation and the direct-current magnetic field, the adhesive is heated, such that the bonded articles, optionally with the application of mechanical force, may be separated one from the other.
In many sectors of industry, in particular in the metals processing industry, such as the automotive industry, in commercial vehicle construction and the supply industry thereof, also in the manufacture of machinery and household appliances, and also in the construction industry, identical or different metallic and non-metallic substrates are increasingly commonly being adhesively or sealingly bonded together. This type of joining of components is increasingly replacing conventional joining methods such as riveting, bolting or welding, because adhesive bonding/sealing offers a range of technical advantages. In contrast to the case of traditional joining methods such as welding, riveting, bolting, the problem of detaching and separating adhesively bonded components has in many cases not yet satisfactorily been resolved.
EP-A-735121 describes a portion of adhesive film for providing an adhesive bond which leaves no residues, causes no damage and is detachable consisting of a double-sided adhesive film having a projecting tab of the adhesive film, by means of which tab the adhesive bond may be detached by pulling in the direction of the plane of the bond. This process may, however, only be used if the adhesive layer of the adhesive film is a pressure sensitive adhesive. However, only low tensile or peel strengths may be achieved with such adhesive bonds, such that this method may only be used in household applications for attaching small articles such as hooks and the like.
DE-A-4230116 describes an adhesive composition containing a blend of an aliphatic polyol with an aromatic dianhydride. This adhesive composition permits dissolution of the adhesive bond in aqueous alkaline systems, with soda solution or alkali metal hydroxide solutions specifically being mentioned. It is proposed to use these aqueous alkaline soluble adhesives to facilitate production of magnetic parts and other small parts, wherein the adhesive is only intended to be used to provide auxiliary bonds during materials processing. Very similar adhesives are also known as labeling adhesives, which permit the removal of labels from beverage bottles and similar containers.
DE-A-4328108 describes an adhesive for floor coverings and a process for detaching these adhesively bonded floor coverings using microwave energy. To this end, the adhesive should be electrically conductive and be softenable by a microwave apparatus. Specifically, said document proposes solvent free contact adhesives based on (aqueous) polymer dispersions which contain copper powder or aluminum powder. According to the teaching of this document, the adhesively bonded pieces of floor covering are placed in a microwave apparatus to detach the adhesive bond, such that the adhesive layer may be softened and the pieces of floor covering may be peeled off manually once the adhesive layer has softened.
WO 94/12582 describes a pressure sensitive adhesive based on a mixture of an aqueous polymer dispersion and an adhesive dissolved in an organic solvent together with tackifying components and thickeners. This pressure sensitive adhesive has a constant adhesive strength over a wide temperature range and permits mechanical separation of the adhesive bonds. It is stated that these adhesive bonds are suitable for bonding insulating and/or decorative surface parts, for example isulating materials or plastics films.
DE-A-19526351 describes a dissolving gel for lacquers, paints and adhesives based on organic solvents with the addition of wetting agents, thickeners and other conventional agents. One specific application which is mentioned is use as a stripper for removing two component lacquers. Although it is mentioned that such mixtures may also be used for two component adhesives, no specific details are stated with regard to detaching such adhesive bonds. Similarly, WO 87/01724 describes a composition for removing cured polysulfide sealants or coatings. In this case, an alkali metal thiolate or ammonium thiolate based on alkyl or phenyl thiolates is dissolved in a solvent or solvent mixture consisting of dimethylformamide or dimethylacetamide or a mixture thereof with aromatic solvents such as toluene or xylene and applied onto cured polysulfide sealants or coating materials, such that these may then be removed from their substrates, such as aircraft tanks. No details are stated with regard to detaching adhesive bonds.
In the paper Reversible crosslinking in epoxy resins, Journal of Applied Polymer Science, 39, 1439-1457 (1990), V. R. Sastri and G. C. Tesoro describe epoxy resins with differing epoxy equivalents which are crosslinked with 4,4′-dithioaniline. It is suggested in this paper to grind the crosslinked resin into particles of a size of 600 μm. This finely ground powder is then refluxed in a solution of diglyme, hydrochloric acid and tributyl phosphine until the ground resin has dissolved. The same authors make similar disclosures in U.S. Pat. No. 4,882,399. Neither document gives specific details with regard to detachable adhesive bonds.
WO 99/07774 describes adhesives, in which at least one synthesis component contains di- or polysulfide bonds and which, once cured, may be detached by applying solutions of cleaving agents based on mercapto compounds. In this manner, it is possible to separate adhesively bonded components by chemical means in the glueline. According to the teaching of this document, the cleaving agent may also be incorporated into the adhesive formulation in a form which is inert at room temperature, wherein cleavage may proceed once the reagent has been activated at elevated temperature. Specific embodiments of this inert form of the cleaving agent are not stated. Although using cleaving agents containing solvents permits the detachment of adhesive bonds, it is desirable to be able to dispense with cleaving agents containing solvents because    this approach is very time-consuming due to the contact time with the cleaving agents which is determined by diffusion processes,    handling of cleaving agents containing solvents should be avoided on environmental protection grounds.
DE-A-19924138 describes adhesive compositions, which contain nanoscale particles having ferromagnetic, ferrimagnetic, superparamagnetic or piezoelectric characteristics in the binder system. These adhesive compositions are suitable for producing detachable adhesive bonds. On exposure to electromagnetic radiation, these adhesive compounds may be heated to such an extent that the adhesive bond may readily be detached.
DE-A-35 01 490 describes glazing adhesively bonded into an automotive body using an elastomeric crosslinked adhesive. This glazing has on its surface in the bonding area a conductive track provided with electrical terminals, which track bears on the side facing towards the adhesive a release layer of a thermally fusible material, such as soft solder or a thermoplastic. The adhesive bond is detached by supplying electricity to the conductive track, which heats up, melts the release layer and the glazing may be detached from the body.
EP-A-0521825 describes a detachable adhesive bond, in which the bonded components are joined by a bead of adhesive placed between them. This bead of adhesive contains a two-dimensional thermoplastic release element. This thermoplastic release element contains intrinsically conductive polymers, electrically conductive carbon blacks, graphite, metal powder, metal fibers or metal needles, metal coated fillers, metal coated glass microbeads, metal coated textile fibers or mixtures of these materials. When the adhesive bond is heated by electricity or input of radiation, this thermoplastic release layer is softened, such that the joined parts may be separated from each other mechanically. EP-A-521825 specifically proposes using such detachable adhesive bonds for direct glazing in automotive construction.